Mole guidance system

ABSTRACT

A subterranean missile is equipped with three mutually orthogonal coils in its body and a fourth suitably fixed in an articulatable steering member. The missile is guided along any desired underground trajectory defined with reference to a coordinate system including the plane of a dipole-quadrupole antenna system laid on the ground above. Voltages induced in the noncenterline body coils are used in a closed feedback loop to maintain the resultant magnetic field circularly polarized at the mole location. Heading errors are then revealed as a voltage phasor on the centerline coil. Rotation of the steering member sufficiently to bring the voltage induced in its coil into phase with the centerline coil voltage also aligns the steering member such that its articulation will reduce the heading error to zero. Any new trajectory may be defined as a suitable new voltage added to the centerline coil voltage.

United States Patent [72] inventor James C. Coyne New Providence, NJ.1211 Appl. No. 792,393 [22] Filed Dec. 27, 1968 [45] Patented June 29,1971 [73] Assignee Bell Telephone Laboratories, Incorporated MurrayHill, NJ.

[54] MOLE GUIDANCE SYSTEM 10 Claims, 13 Drawing Figs.

[52] 11.5. CI 175/26, 175/45. 175/61, 175/73 [51] Int. Cl E2lb 7/04 [50]Field oiSearch 175/26, 45, 61, 73; 340/183, 195

,[56] References Cited UNlTED STATES PATENTS 2,845,613 7/1958 Pawley340/183 3,282,355 11/1966 Henderson 175/61 3,307,164 2/1967 Zimmer340/195 3,326,008 6/1967 Baran et al 175/26 PROCESSING cmcumzv L3,461,979 8/1969 Newfarmer ABSTRACT: A subterranean missile is equippedwith three mutually orthogonal coils in its body and a fourth suitablyfixed in an articulatable steering member. The missile is guided alongany desired underground trajectory defined with referenceto a coordinatesystem including the plane of a dipole-quadrupole antenna system laid onthe ground above. Voltages induced in the noncenterline body coils areused in a .Closed feedback loop to maintain the resultant magnetic fieldcircularly polarized at the mole location. Heading errors are thenrevealed as a voltage phasor on the centerline coil. Rotation of thesteering member sufficiently to bring the voltage induced in its coilinto phase with the centerline coil voltage also aligns the steeringmember such that its articulation will reduce the heading error to zero.Any new trajectory may be defined as a suitable new voltage added to thecenterline coil voltage.

PATENIEU JUH29 I97! SHEET 1 BF 6 lNl/E/VTOR By J. C. COVNE w 15 A TTORNEV PATENlEnJuuzslsn 3,589,454

sum 2 or 6 DIRECTION OF ARTICULATION FIG. 2

PATENTED JUN29I97I 3589.454

SHEET 5 [1F 6 FIG. 5A FIG. 5C

(TOP vIEw FROM I -(TOP vIEw) ANTENNA PLANE) XOIYOZO X TARGET POINT oa o)0 X! DESIRED I TRAJECTORY DESIRED TRAJEcTow PRESENT POSITION I x,,v,, ZII W Y FIG. 50 FIG. 5B (sIoE ELEVATION) (SIDE ELEVATION TAKEN IN FLUXPLANE) DESIRED TRAJECTORY MOLE GUIDANCE SYSTEM This invention relates tosubterranean missiles, and specifically concerns a guidance system forsuch devices.

BACKGROUND OF THE INVENTION In the patent application ofCoyne-Elia-Southworth, Ser. No. 764,750, filed Oct. 3, I968, there isdescribed a system for continuously detecting the position and attitudeof a subterranean missile or Mole with respect to a fixed coordinatesystem relative to a pair of antennae laid on the ground over thedesired trajectory. In that system, the position coordinates of the molein the magnetic flux plane of the antennae as well as its pitch, yaw,and roll angles all are continuously calculated from voltages detectedin a trio of mutually orthogonal magnetometers fixed in the mole body.From these voltages appropriate steering instructions are given the moleto cause it to assume the desired heading.

This earlier system recognized the advantages of a dipole quadrupoleantenna. For example, calculation of the mole depth is a simple functionof the antenna half spacing. Mole distance from the center wire,measured in multiples of the antenna half spacing, is equal to the ratioof dipole to quadrupole field strength, as measured by magnetometers inthe mole at the mole location. Also, the polar angle at the origin isexactly equal to the included angle between the field vectors, asmeasured by the mole magnetometers.

While thus constituting a fully workable location detection scheme, theearlier system nevertheless has several drawbacks. For one, thecomputations of position and heading data requires extensive logiccircuitry. More importantly, the system does not provide for the mole toseek out a prescribed trajectory. Moreover, the trajectory itself is notsuperimposable onto the system.

Accordingly, the principal object of this invention is to simplify thecontrol of a subterranean missile in its movement from one point toanother along a prescribed path.

Another object of the invention is to automatically guide a mole to aspecified terminal point within a reference frame defined by a pair ofmagnetic field-generating antennae.

A further object of the invention is to achieve an automatic moleguidance system in which the terminal point whichthe mole seeks can berespecified at will.

SUMMARY OF THE INVENTION The present invention, broadly, is grounded inthe recognition of the advantages of a rotating magnetic field vector atthe mole location. Mole heading errors can be directly detected by phasecomparison of the induced voltages in an axial magnetometer in the molebody with that in a magnetometer mounted in an articulatable steeringsurface.

The heading error is the angle between the mole longitudinal axis and aline parallel to the magnetic field propagating antenna. The headingerror is zero when the mole axis is parallel to the antenna wire. Inthis position, no voltage is included in the axial magnetometer; butwith heading error from any mole orientation whatsoever, a small butdetectable voltage is induced in the axial magnetometer. The trajectorywhich would reduce the heading error to zero lies in the plane definedbythe mole axis and a line through the mole parallel to the antenna wire.Thus, a steering surface articulated in that plane will cause theheading error to reduce to zero. The steering surface is placed in thatplane by manipulating the surface until, pursuant to the invention, theaforementioned induced voltages are in phase with each other.

The rotating magnetic field can be produced by any convenient antennaconfiguration energized to produce a rotating vector. An antennaconfiguration which is preferred because of the simple geometricrelationships which it provides, is the dipole-quadrupole scheme of theaforementioned patent application Ser. No. 764,750. For that antennaconfiguration, as well as for certain others, a preferred antennaenergization is with signal currents of the same frequency but differingby a phase factor.

For heading error correction pursuant to the invention, the magneticfield vector must merely rotate. It need not describe a circular pathand, in fact, can be widely elliptical with no performance penalty.

However, pursuant to a further and major facet of the invention, a fieldmaintained in circular polarization at the mole location permits controlof the mole s position in three-dimensional space through the expedientof heading control. A mole trajectory originating from its presentposition and extending to a desired terminal point is specifiedaccording to this aspect of the invention by summing an appropriatevoltage phasor with the output signal of the mole axial magnetometer.The specified trajectory can be one parallel to the antenna, ordiverging or converging with respect thereto. The superimposed voltagephasor plus the induced voltage in the axial magnetometer produces avoltage phasor output identical to the phasor that would have beeninduced had the antenna been actually moved to a position directly aboveand parallel to the chosen trajectory. Steering then proceeds in exactlythe same manner as above. The steering surface is manipulated untilvoltages in its magnetometer and in the mole axial magnetometer are inphase; and the surface is articulated so as to keep them in phase. Theheading error to be reduced to zero now, however, is with respect to theline between the moles present position and the desired terminal point.

One feature of the invention, therefore, is the use of a rotatingmagnetic vector which provides a phase comparison way to find thecorrect position of a mole steering surface to counteract any headingerror.

A further feature of the invention involves defining new moletrajectories whichthe mole seeks out by heading control using the samephase comparison methods, thereby providing the capability of steeringtoward any desired terminal point.

The invention, its further objects, features and advantages will befully apprehended from a reading of the description to follow of anillustrative embodiment.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic perspective diagram ofthe mole and antenna system;

FIG. 1A is a perspective diagram depicting certain geometric relations;

FIG. 2 is a schematic perspective diagram of the mole articulating tail;

FIGS. 3, 3A, 3B, and 3C are schematic diagrams, taken with the plane offlux lying in the paper, and showing the mole and tail in variousworking positions with resulting voltages induced in certain coils;

FIG. 4 is a diagram, taken in the flux plane of the antenna system andshowing certain critical geometric relationships between the mole and amagnetic field vector;

FIGS. 5A, 5B, 5C, and 5D are diagrams illustrating the criticalgeometric relationships of mole heading in terms of a new desiredtrajectory; and

FIG. 6 is a block diagram of an overall mole guidance system pursuant tothe invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT The mole workingenvironment is shown schematically in FIG. I. Current-carryingconductors in the form of a dipole antenna 10 and a quadrupole antenna11 are laid on the ground over the desired route. Their construction andmounting details are described in the aforementioned patent applicationSer. No. 764,750. AC voltages are applied to antennae 10, 11 by poweramplifiers 12, 13, respectively. The voltages are maintained at the samefrequency, for example, 5 kHz., but, importantly, differ in phase in amanner to be described.

FIG. 2 shows a mole l4 equipped with suitable components for use in thepresent guidance system. Mole 14 consists of a body 15 and a tailassembly 16. Assembly 16 is mounted for rotational movement a full 360about the centerline axis 17 of body 15. The tail itself, designated 18,is pivotable about a fixed clevis pin 19 which is perpendicular to axis17. Control of rotation of assembly 16 and articulation of tail 18 areaffected by conventional servomechanisms consisting of rotary actuator20 and articulation actuator 50 which are shown in FIG. 6 schematically.

FIGS. 1 and 2 show three magnetometers such as induction coils 21, 22,23, fixedly mounted in mole 14. Their sensitive axes A, B, C aremutually orthogonal. As installed, axis A of coil 21 is coincident withthe mole centerline axis 17. A fourth magnetometer such as coil 24 ismounted in tail 18 with its sensitive axis D normal to the extendedcenterline axis 17 and also normal to clevis pin 19. Electricalconnections run from each magnetometer to the processing circuitry whichwill be fully explained with reference to FIG. 6.

CIRCULAR POLARIZATION As two distinct magnetic fields exist in the soilbulk, the net field illustrated in FIG. 4 at a specified point and timeis given by the vectorial addition of the separate fields, the latterdenoted D, Q. Consider their vectorial addition for an arbitrarilychosen position of mole 14 as seen in the flux plane. The field vectorsD, Q have peak values ofD 6: respectively. As the fields are out ofphase, the peaks occur at different times. At the given time, theirinstantaneous values are In FIG. 4, vector Twas arbitrarily taken at itsmaximum value D. The vector resultant of the instantaneous vectors I6:is?

It is seen that b rotates in space with passh g time, and in generaldescribes an ellipse. Resultant vector b passes through the sensitiveaxes, A, B, C, D of coils 21-24; 21rd voltages are induced in each coil.When the component ofb along a coil is maximum, the induced voltage inthat coil is a maximum. These sinusoidal voltages are denoted V V V V.respectively for the coils 21-24; voltages V and V are schematicallydepicted in FIGS. 3A and 38.

It may be desired, as for certain purposes of the present invention, toprovide a circularly polarized magnetic field, i.e., to causebto rotatewith constant angular velocity and constant magnitude. The followingconsiderations then apply. FIG. 4 illustrates that D and 6 differ indirection by the geometric angle I Further, d and q differ in phase bythe angle B which is the phase difference between the applied sinusoidalcurrents i and i to antennae l0, l1. Omitting prqgfihe conditions forcircular polarization are:

Such conditions are maintained, for example, by monitoring the voltagesinduced in coils 22, 23 whose axes B, C lie in or near the place offlux. When the field is circularly polarized, the voltages V V inducedby b coils 22, 23 have equal magnitude and 90 phase difference. This isachieved, for example, by fixing the dipole antenna current i and byproviding means for controlling the phase and magnitude of thequadrupole current i, so that the field at the mole location iscircularly polarized.

If now the mole undergoes a position change, the field becomes slightlyelliptical, and the output voltages V V;, of magnetometers 22, 23 willchange slightly in both amplitude and phase. A small correction of bothmagnitude Al and phase AB of i, is required to reestablish the circularfield and is achieved in the antenna current regulator 60 shown in FIG.6.

It should be understood that the invention broadly involves the use of arotating magnetic field vector. Accordingly, while the antenna signalfrequencies have in the illustrative embodiment been specified to be thesame while differing by a phase factor, other schemes exist forrealizing a rotating vector. Also, while the dipole-quadrupole antennadescribed in applicants preferred mode, other antenna structures can beenvisioned for realizing the rotating vector. The advantages of thedipole-quadrupole configuration are that with the driving currentdescribed earlier and circular polarization, a simple determination ofmole position is afforded. That is, the ratio of quadrupole antenna peakcurrent to dipole antenna peak current equals the distance from theantenna center wire 11a (measured in multiples of the antenna halfspacing). Also, the supplement of the phase difference between dipoleand quadrupole antenna currents equals the polar angle. This point andothers will now be expanded.

AUTOMATIC I-IEADING CONTROL The rotating magnetic vector; pursuant to amajor facet of the invention, enables finding the correct position of asteering control surface such as tail 18 to counteract any headingerror.

The coordinate system notations used below and alluded to elsewhere alsobear summarization at this point. The XYZ system is with reference tothe stationary antenna as seen in FIGS. 1 and 1A, for example; theX-axis is coincident with the center wire 11a, and the XY plane isdefined by the antenna wires 10, 11, 11a. Positions in the soil bulk aremeasured conveniently with reference to the XYZ system. The XYZ systemis a coordinate system located with reference to the XYZ system, andcomes into play when a new trajectory is to be specified in a manner tobe described. The X"Y"Z" system moves with the mole, its axes beingalways parallel to the respective axes of the XYZ system.

Heading error is any orientation of mole 14 which brings the centerlineaxis 17, and the coincident magnetometer axis A, out of parallelism withthe X axis, as illustrated in FIG. 1A. In the earlier patent applicationSer. No. 764,750, heading error included a pitch component b and a yawcomponent 0. Ascertaining these angles required substantial addedequipment complexity. As will shortly be seen however, the presentinvention renders it completely unnecessary, in correcting for headingerror, to calculate the values of either; nor is it necessary to computea roll angle.

In FIGS. 1 and 3 the mole 14 is depicted as being on trajectory, movingtoward a target point X Y Z with zero heading angle. Thus, mole 14 isdirectly under the antenna center wire 11a. Rarely in practice, however,is the mole ideally positioned.

FIG. 1A illustrates the more usual situation. The mole is shown asdeviating from the desired heading, namely the X" axis. This headingdeviation is measured by the various angles shown, and now defined.

called the pitch angle, is the angle between the mole axis 17 and itsprojection on the XY" plane.

6, called the yaw angle, is the angle between the projection of the moleaxis 17 on the XY" plane and the X axis.

H, called the heading angle, is the angle between the moles axis 17 andthe X axis.

M is the angle between the projection of the moles axis 17 on the Y"Zplane and the negative Z" axis.

6 and are related to H and M by the following equations: are

Il d 2 Accordingly, FIG. 3A depicts the mole as deviating in positionand also from the desired heading by an arbitrary heading error angle Hwhich, it will be understood, consists of a pitch and a yaw component.The object is to find a rotational position for tail 18 such that, witharticulation, the mole follows a trajectory that not only reduces H, butreduces H to zero.

Consider that when the mole tail 18 is articulated, the mole trajectorylies in a plane defined by the mole centerline axis and the axis of taill8. The heading error angle H can go to zero only if said plane alsoincludes the X axis. Accordingly, the problem reduces to articulatingtail 18 in the plane defined by the X" axis and the mole centerline axis17.

It can be seen by reference to FIGS. 3A-3C that the correct rotationalposition of tail 18 occurs when the voltage induced by Fin coils 21 and24 are in phase. With any heading error, axis A of coil 21 has aprojectable component in the plane of flux, as seen in FIG. 3A. In suchcase, as seen in the bottom diagram of FIG. 3A vector? induces a voltageV in coil 21 which peaks when the component of vector b on axis A ismaximum. Ifthe field were exactly circular this would occur when vectorband axis A were coincident. Tail coil 24 when viewed in the flux planehas a voltage V induced by vector? Owing to the initial random rotationof tail 18 with respect to the mole body, however, the induced voltagesin coils 21 and 24 differ by a phase factor. This phase difference ismonitored by suitable circuitry such as depicting in FIG. 6. Rotaryactuator then is servoed until the phase difference is zero as shown inFIG. 3B. Coil axes A and D now lie in a plane that in cludes theX"-axis. Articulation of tall 18 around pin 19 at this time as depictedin FIG. 3C, will place the mole on a trajectory that at some time isassured of bringing the mole parallel to the X-axis, thus reducing theheading error angle H to zero.

When the tail 18 articulates, the magnitude of voltages induced in coil24 will decrease, but the phase will remain unchanged. Further, as themole l4 proceeds along its curved trajectory in seeking a zero headingangle error, the phase of voltage V, will continue to remain unchangedprovided no disturbances roll the missile or divert it out of thedesired trajectory plane.

If the mole is diverted, however, then the voltage phasers V and V, willbe once again out of phase, and the inventive heading correctionprocedure will continuously correct. If the articulation angle is toolarge that the rotary unit 20 cannot turn the tail against soil forces,the tail is straightened and then rotated until V and V, are in phase;and then the tail is rearticulated. If the articulation angle is notgreat, a new rotary position of tail assembly 16 can be sought withoutstraightening the tail.

The articulation angle itself is chosen to the desired turning curvatureand does not affect the phase matching of voltages V, and V The changein magnitude of voltage V, as tail 18 articulates does provide a meansof measuring the articulation angle, however.

The voltages V and V, are depicted in FIGS. 3A3C as being equal inmagnitude merely for ease of illustration. They need not be; and inpractice rarely are.

It should be understood that the critical inventive point thus far isthe correction of heading angle errors by phase comparison techniques,in which the phase of the voltage induced in the axial magnetometer 21by a rotating magnetic field vector is compared to the phase of thevoltage induced in the magnetometer 24 which is fixed in the steeringmember in a predetermined orientation. This orientation is with respectto the sensitive axis D and the corresponding steering attitude whichthe steering member exhibitsor as in the instant .embodiment, wouldexhibit, if articulated. Accordingly it is clear that the phasecomparison does not require that there be zero phase difference betweenthe induced voltages. It requires only that the phase difference beknown which corresponds to the orientation of the steering membermagnetometer axis D (and thus also the steering member itself) that willreduce the heading angle error to zero. To this end, the choice of azero phase difference is convenient.

HEADING CONTROL PLUS POSITIONAL CONTROL In the system so far described,control of the moles heading angle enables the moles centerline axis 17to be kept parallel to the antenna center wire 110. In many fieldsituations, heading control in the above terms with respect to thestationary center wire would suffice as a complete guidance system if,among other considerations, the mole is originally planted parallel tothe antenna center wire and if further the trajectory is not long. Thereare, however, an infinite number of lines parallel to the center wire,any one of which could be assumed by the mole centerline axis to satisfythe requirement of zero heading angle error. Accordingly, the likelypositional error at the terminal point increases with increasingtrajectory length until it becomes intolerably large.

Pursuant therefore to a further major aspect of the invention,positional control is incorporated into the guidance scheme by causingthe system to guide-again, by heading control-along any new chosenstraightline path from the mole, and not just a path parallel to theantenna center wire. The new path advantageously can be one which linksthe mole and any desired target point.

Such target point normally is directly beneath the center wire andrepresents the terminal point of mole travel. The target point can,however, be both horizontally and vertically removed from the nominalterminal point directly below the center wire end. Further, the targetpoint can be respecified any number of times.

Implementation of position control is depicted in FIG. 6; and involvesgenerating a sinusoidal voltage, denoted R cos (wt-W The latter voltagehas a magnitude and phase equal to the polar coordinates R :11, (FIG.5B) of the target point X,,Y,,Z (FIG. 5A). The quantities R, and th areset, either manually or otherwise, into a phase and magnitude control65.

Control 65 derives another input directly-from the quadrupole antenna11. Antenna current regulator 60, as already described, monitors thevoltages V and V induced in coils 22, 23 and adjusts the antenna currenti, to values that maintain circular polarization.

The sinusoid R, cos (wt-i11 is then added in summing amplifier 70 to thesinusoidal quadrupole antenna voltage R cos (wt-PB). The gain ofamplifier 70 is proportional to l/XX,,, the forward distance of thetarget point.

The summing amplifier 70 output is phase shifted in circuit 71 by thefactor 3(1r/2=B) to place the zero phase reference on the --Z, axis; andthen added in summing amplifier 73 to the voltage induced inmagnetometer 21. The latter has been normalized in circuit 72 by thefield strength as measured by the RMS voltage V Normalization can aswell be achieved with the RMS voltage V in the same fashion. The outputof the normalizing operation is a sinusoidal voltage whose amplitude isthe heading angle I-I measured in radians, and whose phase is the angleM as seen in FIGS. 1A and 5B.

The output of summing amplifier 73 yields a new voltage phasor, denotedI-I cos (wt+M) whose amplitude H is the heading angle of the mole withrespect to the new desired trajectory X seen in FIGS. 5A-5C, and whosephase is the angle M shown in FIG. 5D.

The output of summing amplifier 73 is fed to two comparators 74 and 75.In comparator 74 the phase of the voltage in coil 24 is compared withthe phase of output of summing amplifier 73. The difference error signalAM is used to drive the rotary actuator 20 until the error signal goesto zero. This operation places the tail 18 in a position such that whenarticulated the heading angle with respect to the X axis will go tozero.

In the second comparator 75 the amplitude of the output of summingamplifier 73 is compared with two reference voltages, H max. and 1-1min. These are, for example, set in manually and represent the dead zoneof the on-off control of the tail articulation actuator 50. When l-I'exceeds H max. the valve motor is turned on, opening valve 81 tohydraulic supply pressure which drives actuator 50 through suitablehydraulic connections 82. 'With tail 18 articulated, the mold assumed atrajectory that begins to reduce the heading angle H toward zero. WhenI-I' drops below 1-1 min., comparator 75 signals valve motor 80 toremove supply pressure; and the fluid pressure is relieved as throughreturn line 83. The mole tail straightens out in about a tail length oftravel.

The capability of specifying and respecifying the mole target point hasnumerous highly useful ramifications. It is, for example, possible tocorrect for the natural tendency of the mole to surface due to thegreater strength of soil below the mole bottom, simply by moving thetarget point closer in toward the mole. Obstructions such as rocks,drain pipes and the like, whose locations are known, are readily avoidedby specifying a series of safe trajectories. It further is possible tosteer straight paths under bushes, for example, where laying the antennain a straight path is precluded. Also, the antenna does not require anyrelocation in the process of specifying the trajectory. Additionally,the mole can be caused to fare into a desired trajectory in a sweepingarc, simply by leaving the gain of amplifier 70 constant. Aiming at afixed point in contrast requires changing of the amplifier gain.

Numerous additional advantages, variations and embodiments of theinvention will be readily envisioned by persons skilled in the art. Thespirit of the invention is limited only by the scope of the claims tofollow.

What i claim is: t l. A method for controlling the heading of amissilelike burrowing device having a longitudinal axis, within a soilbulk comprising the steps of:

generating in said bulk a rotating magnetic vector that bears a knowngeometric relationship to a desired trajectory with respect to whichsaid heading is to be controlled,

detecting the phase angle of said vector when it aligns with the devicelongitudinal axis, and

manipulating a steering member that is movably connected to the moletoan attitude of known relation to said phase angle such that the newdevice trajectory will include a heading direction that is perpendicularto said magnetic vector.

2. A method for controlling the heading of a missilelike burrowingdevice having a longitudinal axis, with respect to a desired trajectory,comprising the steps of:

generating a rotating magnetic field vector at the mole location. saidvector bearing a known geometric relationship to said trajectory,detecting the phase difference between signals induced by said vector intwo magnetometers whose sensitive axes lie respectively along the devicelongitudinal axis, and selectively fixed within a steering member thatis movably connected to the mole, and

manipulating said steering member until said phase diffcrence is sspecified value corresponding to a steering attitude that will bringsaid mole into said desired trajectory.

3. A heading control method pursuant to claims l or 2, wherein saidvector generating step comprises generating a circularly polarized fieldvector.

4. A heading control method pursuant to claims 1 or 2,

wherein said generating step further comprises generating a circularlypolarized magnetic field; and said method comprises the further steps ofgenerating a substantially sinusoidal signal having a magnitude andphase proportional to polar coordinates of a specified target point,adding said phasor to the induced voltage phasor ofsaid longitudinalmagnetometer, and adjusting the resulting phssor by a factorproportional to the present device position, thereby spceiiying a newdevice trajectory.

5. A heading control system for a steerable, missilelike burrowingdevice operating within a soil bulk comprising:

means for generating a rotating magnetic field vector at the devicelocation, a first magnetometer mounted with its sensitive axissubstantially coincident with the device longitudinal axis, a secondmagnetometer having its sensitive axis mounted in a predeterminedorientation within a positionable steering surface, means for detectingthe phase difference between the voltages induced by said rotating fieldin said magnetometers, means for positioning said steering surface toreduce said phase difference to a specified value, and means foraffecting a steering attitude in said steering sur= fees such asmaintains said phase difference at said specified value. 6. A headingcontrol system pursuant to claim 5, wherein said generating meansgenerates a circularly polarized magnetic field at the device location.

7. For a missilelike burrowing device having a centerline axis and asurface placeable into a steering attitude with respect to said axis, aguidance system comprising in combination:

means for generating and maintaining at the mole location a circularlypolarized magnetic field whose rotating vector is perpendicular to areference trajectory;

means for detecting the phase of first signals developed by said fieldin the direction of said centerline axis;

means for detecting the phase of second signals developed by said fieldin the direction of the present permissible steering attitude of saidsurface;

means for rotating said surface about said centerline to bring saidsignals into phase; and

means for thereafter placing said surface into a steering attitude suchas maintains said signals in phase, whereby mole heading errors withrespect to said reference trajectory are corrected.

8. For a mole having a centerline axis and a steering tail rotatableabout said axis and articulatable with respect to a pivot pinperpendicular to said axis, a guidance system comprising in combination:

means for generating and maintaining at the mole location a circularlypolarized magnetic field whose rotating vector is perpendicular to areference trajectory;

means for detecting the phase of first signals developed by said fieldin the direction of said centerline axis;

means for detecting the phase of second signals developed by said fieldin the direction which said tail would move if then articulated;

means for rotating said tail to bring said signals into phase;

and

means for articulating said tail to bring said mole into a trajectorythat reduces the magnitude of said first signals to zero.

9. A mole guidance system pursuant to claims 8, 9, or 10,

wherein the means for generating said circularly polarized fieldcomprises a dipole and a quadrupole antenna loop,

driven by AC currents of the same frequency but differing by a phasefactor;

means for monitoring the phase and magnitude of the dipole andquadrupole fields at the mole location; and

means for adjusting the antenna driving currents and magnitudes so as tocreate a resultant vector at the mole location which rotates withconstant magnitude and angular velocity. 10. For a mole having acylindrical elongated body, a trio of magnetometers fixed in said body,with their sensitive axes mutually orthogonal, a first said magnetometerbeing axially coincident with the centerline of said body, a steeringtail rotatable about said centerline and articuiatable about a pivot pinnormal to said centerline. and a fourth magnetometer fixed in said tailat right angles to said pivot pin, a guidance system comprising incombination:

means for generating and maintaining at the mole location a circularlypolarized magnetic field whose rotating vector bears a known geometricrelation to a reference trajectoy;

means for detecting the phase difference between voltages developed bysaid field in said first and fourth magnetometcrs',

means for rotating said tail about said centerline to reduce said phasedifference to zero; and

means for thereafter articulating said tail in a direction that placessaid mole into a trajectory which will reduce the magnitude of saidfirst magnetometer voltages to zero.

1. A method for controlling the heading of a missilelike burrowingdevice having a longitudinal axis, within a soil bulk comprising thesteps of: generating in said bulk a rotating magnetic vector that bearsa known geometric relationship to a desired trajectory with respect towhich said heading is to be controlled, detecting the phase angle ofsaid vector when it aligns with the device longitudinal axis, andmanipulating a steEring member that is movably connected to the mole toan attitude of known relation to said phase angle such that the newdevice trajectory will include a heading direction that is perpendicularto said magnetic vector.
 2. A method for controlling the heading of amissilelike burrowing device having a longitudinal axis, with respect toa desired trajectory, comprising the steps of: generating a rotatingmagnetic field vector at the mole location, said vector bearing a knowngeometric relationship to said trajectory, detecting the phasedifference between signals induced by said vector in two magnetometerswhose sensitive axes lie respectively along the device longitudinalaxis, and selectively fixed within a steering member that is movablyconnected to the mole, and manipulating said steering member until saidphase difference is a specified value corresponding to a steeringattitude that will bring said mole into said desired trajectory.
 3. Aheading control method pursuant to claims 1 or 2, wherein said vectorgenerating step comprises generating a circularly polarized fieldvector.
 4. A heading control method pursuant to claims 1 or 2, whereinsaid generating step further comprises generating a circularly polarizedmagnetic field; and said method comprises the further steps ofgenerating a substantially sinusoidal signal having a magnitude andphase proportional to polar coordinates of a specified target point,adding said phasor to the induced voltage phasor of said longitudinalmagnetometer, and adjusting the resulting phasor by a factorproportional to the present device position, thereby specifying a newdevice trajectory.
 5. A heading control system for a steerable,missilelike burrowing device operating within a soil bulk comprising:means for generating a rotating magnetic field vector at the devicelocation, a first magnetometer mounted with its sensitive axissubstantially coincident with the device longitudinal axis, a secondmagnetometer having its sensitive axis mounted in a predeterminedorientation within a positionable steering surface, means for detectingthe phase difference between the voltages induced by said rotating fieldin said magnetometers, means for positioning said steering surface toreduce said phase difference to a specified value, and means foraffecting a steering attitude in said steering surface such as maintainssaid phase difference at said specified value.
 6. A heading controlsystem pursuant to claim 5, wherein said generating means generates acircularly polarized magnetic field at the device location.
 7. For amissilelike burrowing device having a centerline axis and a surfaceplaceable into a steering attitude with respect to said axis, a guidancesystem comprising in combination: means for generating and maintainingat the mole location a circularly polarized magnetic field whoserotating vector is perpendicular to a reference trajectory; means fordetecting the phase of first signals developed by said field in thedirection of said centerline axis; means for detecting the phase ofsecond signals developed by said field in the direction of the presentpermissible steering attitude of said surface; means for rotating saidsurface about said centerline to bring said signals into phase; andmeans for thereafter placing said surface into a steering attitude suchas maintains said signals in phase, whereby mole heading errors withrespect to said reference trajectory are corrected.
 8. For a mole havinga centerline axis and a steering tail rotatable about said axis andarticulatable with respect to a pivot pin perpendicular to said axis, aguidance system comprising in combination: means for generating andmaintaining at the mole location a circularly polarized magnetic fieldwhose rotating vector is perpendicular to a reference trajectory; meansfor detecting the phase of first signals developed by said field in thedirection of said cEnterline axis; means for detecting the phase ofsecond signals developed by said field in the direction which said tailwould move if then articulated; means for rotating said tail to bringsaid signals into phase; and means for articulating said tail to bringsaid mole into a trajectory that reduces the magnitude of said firstsignals to zero.
 9. A mole guidance system pursuant to claims 8, 9, or10, wherein the means for generating said circularly polarized fieldcomprises a dipole and a quadrupole antenna loop, driven by AC currentsof the same frequency but differing by a phase factor; means formonitoring the phase and magnitude of the dipole and quadrupole fieldsat the mole location; and means for adjusting the antenna drivingcurrents and magnitudes so as to create a resultant vector at the molelocation which rotates with constant magnitude and angular velocity. 10.For a mole having a cylindrical elongated body, a trio of magnetometersfixed in said body, with their sensitive axes mutually orthogonal, afirst said magnetometer being axially coincident with the centerline ofsaid body, a steering tail rotatable about said centerline andarticulatable about a pivot pin normal to said centerline, and a fourthmagnetometer fixed in said tail at right angles to said pivot pin, aguidance system comprising in combination: means for generating andmaintaining at the mole location a circularly polarized magnetic fieldwhose rotating vector bears a known geometric relation to a referencetrajectory; means for detecting the phase difference between voltagesdeveloped by said field in said first and fourth magnetometers; meansfor rotating said tail about said centerline to reduce said phasedifference to zero; and means for thereafter articulating said tail in adirection that places said mole into a trajectory which will reduce themagnitude of said first magnetometer voltages to zero.